The Montana Democrat has infuriated some in his party over the years by walking a centrist political line in a state that regularly backs Republicans for president but has a long history of elevating members of either party to other statewide offices.

But he’s been a consistent political survivor who will likely bequeath his seat to yet another Democrat, former Gov. Brian Schweitzer, should the extremely popular Mr. Schweitzer opt to enter the race.

Senator Baucus recently drew the ire of Democrats, including the campaign organization built by Mr. Obama’s presidential campaign, by calling the implementation of the president’s health-care law a looming “train wreck” and for voting against expanding background checks for firearms sales.

At the same time, Baucus has shown his political mettle in turning back a host of conservative challengers over the past three decades.

“Every six years, Democratic Sen. Max Baucus lands on Republicans’ target list,” wrote the nonpartisan Cook Political Report in an analysis of the race, “and every six years they are disappointed.”

The retirement is less about politics and more about the Montanan’s personal situation, says David Parker, a professor at Montana State University, who is writing a book about the 2012 Senate election in Montana.

Recently remarried and already the Big Sky State’s longest-serving senator at age 71, Baucus has seen many of his longtime colleagues fade from the Senate over the last two terms, Mr. Parker says. Even with $5 million in campaign funds already in the bank, the Senate Finance Committee chairman could have had a grueling campaign on his hands, recalling the bitter and expensive contest between Sen. Jon Tester (D) and then-Rep. Denny Rehberg (R) in 2012.

“You look at what Jon Tester went through,” says Parker, noting that Baucus worked tirelessly to help his friend and colleague. “That was a tough race. Jon worked hard and Denny worked hard. When you’re 71, you might say, ‘Is it worth it to me?’ ”

The politics that may have pushed Baucus to stand down in 2014, from tax reform to guns to the health-care law he helped birth, are murky, but what happens next in Montana is rather straightforward, says Parker.

“If Schweitzer comes in and decides to run, I don’t see a path forward for the Republicans. If he doesn’t? Anything could happen – for both sides,” Parker says. “It all begins and ends with Brian Schweitzer.”

With or without Schweitzer, Democrats see the seat as one they’ll fight to keep.

“Democrats have had a great deal of electoral success in Montana over the last decade, and I am confident that will continue,” said Sen. Michael Bennet (D) of Colorado, the head of his party’s committee charged with electing Democratic senators, in a statement. “Democrats built an unprecedented ground game in Montana in 2012 when Senator Tester was reelected, and we will continue to invest all the resources necessary to hold this seat.”

Republicans, for their part, see Baucus's exit as proof that the president’s health-care law will spell doom for many of the seven Democrats having to defend seats in states won by Republican presidential nominee Mitt Romney.

"Vulnerable Democrats will face voters just as ObamaCare's tax hikes, mandates, fees, penalties, and red-tape bureaucracy take shape over the next eight months, and Senator Baucus' retirement reflects that political reality,” said Rob Collins, executive director of the National Republican Senatorial Committee, in a statement. “The 2014 electoral map is in free-fall for Democrats, who were already facing a daunting challenge."

A survey from the liberal-leaning Public Policy Polling in February found Baucus beating either of the two announced Republican candidates (former state Senate majority leader Corey Stapleton and state Rep. Champ Edmunds) but with more voters viewing Baucus unfavorably than favorably. Moreover, Schweitzer outstripped Baucus in a hypothetical matchup between the state’s deepest-rooted political figures.

While Schweitzer has previously soft-pedaled his ambitions in Washington, National Journal reported that the man famous for his bolo ties was “leaning toward” a Senate campaign.

Parker says the move makes sense for Schweitzer if he does, as has been widely rumored, have designs on the White House.

“If he does have presidential ambitions, [it would help] to do a stint in the Senate,” Parker says. “He has to build that national recognition and national fundraising networks, which is something that is difficult for him to come by coming from such a small state like Montana.”

Baucus has been an effective champion of Montana’s interests in Washington – from exporting beef to Asia to funneling federal dollars into public projects in the Big Sky to the state’s agriculture priorities, the last of which Baucus reportedly raised in the middle of “grand bargain” fiscal negotiations with Vice President Joe Biden in 2011. But he also has many critics who see him as a creature of Washington, a senator whose former staffers are peppered throughout the lobbying industry’s top firms.

"Good bye, Senator K Street,” said Stephanie Taylor, a co-founder of the liberal Progressive Change Campaign Committee, a group that targeted Baucus with negative advertisements after his gun bill vote on background checks.

“Montana will finally have a chance to have a senator with its best interests at heart,” Ms. Taylor said in a statement, “and we hope Brian Schweitzer jumps into the race immediately."